Publication 73
Multi-site, multi-frequency tensor decomposition of magnetotelluric data
Gary W. McNeice and Alan G. Jones
Abstract
Accurate interpretation of magnetotelluric data requires an understanding of the directionality
and dimensionality inherent in the data, and valid implementation of an appropriate method for
removing the effects of shallow, small-scale galvanic scatterers on the data to yield responses
representative of regional-scale structures. The galvanic distortion analysis approach advocated
by Groom and Bailey has become the most adopted method, and rightly so given that the approach
decomposes the magnetotelluric impedance tensor into determinable and indeterminable parts, and
tests statistically the validity of the galvanic distortion assumption. As proposed by Groom and
Bailey, one must determine the appropriate frequency-independent telluric distortion parameters and
geoelectric strike by fitting the seven-parameter model on a frequency-by-frequency and site-by-site
basis independently. Whilst this approach has the attraction that one gains a more intimate
understanding of the data set, it is rather time-consuming and requires repetitive application.
We propose an extension to Groom-Bailey decomposition in which a global minimum is sought to
determine the most appropriate strike direction and telluric distortion parameters for a range of
frequencies and a set of sites. Also, we show how an analytically-derived approximate Hessian of
the objective function can reduce the required computing time. We illustrate application of the
analysis to two synthetic data sets and to real data. Finally, we show how the analysis can be
extended to cover the case of frequency-dependent distortion caused by the magnetic effects of the
galvanic charges.
Source
Geophysics, 66, 158-173, 2001.
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